Traditional vehicle speed control or cruise control systems control power provided by the prime mover to control vehicle speed. For internal combustion engines, speed is controlled by controlling the amount of fuel provided to the engine. Likewise, for electric or fuel cell powered vehicles, the energy supply to the traction motor(s) is controlled to control the speed of the vehicle. These systems can supply a limited amount of negative or braking torque to the drive train to decelerate the vehicle and maintain a set cruising speed. Negative torque may be provided using engine compression braking (which may alter valve timing) for internal combustion engines, or regenerative braking for electric vehicles, for example. However, this limited amount of negative torque is often insufficient to maintain a set cruising speed under various operating conditions, such as descending a long or steep grade.
Prior art speed control strategies provide additional negative torque or braking torque using a graduated or stepped activation of regenerative braking, engine braking, and/or service (friction) braking. While this approach may be acceptable for some applications, transitions between braking modes may be noticeable and objectionable to the driver.